(a) Technical Field
The present disclosure relates to a system and method for activating a fuel cell. More particularly, it relates to a system and method for activating a fuel cell, which can reduce the activation time of a fuel cell and reduce the amount of hydrogen used for the activation, thus improving the productivity of a fuel cell stack and reducing manufacturing cost.
(b) Background Art
A fuel cell stack after assembly is required to be activated before being mounted on a vehicle. Otherwise, electrochemical reaction in the fuel cell does not occur to full extent during initial operation and the overall performance of the fuel cell stack is irreversibly deteriorated.
The activation of the fuel cell provides advantageous effects including, e.g., removal of impurities introduced during the process of manufacturing the membrane-electrode assembly and the fuel cell stack, activation of a catalyst which does not participate in the reaction, creation of a transfer passage of reactants to the catalyst, and hydration of electrolyte contained in an electrolyte membrane and an electrode to ensure a hydrogen ion passage.
As shown in FIG. 1, a conventional activation system includes an electronic load 12 connected between a fuel electrode (“anode” or “negative electrode) and an air electrode (“cathode” or “positive electrode”) of a fuel cell stack 10, a pressure sensor 14 mounted on an outlet of each of the fuel electrode and the air electrode, and a controller 16 controlling the activation of the fuel cell stack 10.
According to a conventional method for activating a fuel cell, after humidified hydrogen and humidified air (oxygen) are supplied to the fuel electrode and to the air electrode, respectively, activation according to a load sequence is initiated under predetermined operating conditions (stoichiometric ratio of fuel gas to air, relative humidity, temperature, and pressure). As an electrochemical reaction occurs in the fuel cell stack according to the load sequence, the amount of water contained in a fluorine polymer electrolyte membrane is increased due to water produced at the air electrode and the humidified water supplied to the fuel gas, thereby performing the activation.
One of the most important requirements for successful activation of the fuel cell stack is to control the percentage of water content at a certain level. That is, the concentration gradient of water contained in the electrolyte membrane of the fuel cell stack must be small.
In an example, as shown in FIG. 2, the load is sequentially applied in the order of (1) OCV (15 min)→(2) 600 mV/cell (75 min)→850 mV/cell (20 min)→(4) 600 mV/cell (30 min) with the steps (3) and (4) repeated three times.
In another example, as shown in FIG. 3, the load sequentially applied in two processes, namely, a pre-process and a post-process. The pre-process is performed in the order of 100→900 mV/cell (each 100 mV/cell−2 min)→1,000 mV (30 min) and the post-process is performed in the order of 900→100 mV/cell (each 100 mV/cell−5 min).
The above-described conventional methods, however, have the following problems. First, it takes a long time to perform the activation due to limitations on utilization of product water of the fuel cell stack. That is, in the water transport according to an increase in load during operation of the fuel cell stack, the amount of water transported by electro-osmotic drag at the fuel electrode becomes larger than the amount of water transported by back diffusion at the air electrode, causing only the water concentration on the surface of the electrolyte membrane at the air electrode to increase. Due to this, water concentration gradient in the electrolyte membrane occurs. It thus takes a long time to perform the activation and the productivity of the fuel cells stack is significantly reduced.
Here, since the Nafion fluorine-containing polymer electrolyte membrane has a hydrophobic PTEE structure on a surface layer thereof and a hydrophilic sulfonic acid structure in an inner layer thereof, a large amount of humidified water supplied to the air electrode as fuel (air) is discharged as it is before it permeates into the inner layer. As a result, a water concentration gradient occurs on the surface of the electrolyte membrane at the fuel electrode and the air electrode. In view of this, since the water produced in the fuel cell stack by the reaction is present in the inner layer of the electrolyte membrane having hydrophilic properties rather than the humidified water supplied as fuel, the product water is more advantageous for the activation than the humidified water.
Second, the amount of hydrogen used is increased. That is, since it takes a long time to activate the fuel cell stack, the amount of hydrogen fuel used is increased, and thus the cost for activating the fuel cell stack is increased.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.